1. Field of the Invention
The present invention relates to a noise suppression processing method and a noise suppression processing apparatus for performing noise suppression processing on an input image signal representing a radiographic image to suppress noise components included in the radiographic image and a program for causing a computer to execute the noise suppression processing.
2. Description of the Related Art
Generally, when radiographic images obtained by computed radiography devices (hereinafter called CR devices) or the like are used in diagnoses, the obtained radiographic images are processed into appropriate images for diagnoses with a desired method such as frequency emphasis processing and gradation processing. The processed images are displayed on CRT monitors as soft copies or output to films as hard copies.
At this time, there is a problem that quantum noise in radiation is noticeable in low density areas of the radiographic images, where radiation doses were small. Therefore, various noise suppression processing methods have been proposed to suppress noise components included in image signals representing radiographic images.
U.S. Patent Laid-Open No. 20020071600 proposes a method for obtaining an original image, in which noise components have been suppressed, by performing multi-resolution decomposition to decompose an original image into a sequence of detail images (band-limited image signals at resolution levels 1-M), obtaining an index value indicating a level of noise suppression based on data representing a radiation dose, suppressing noise components in each of the detail images based on the index value and reconstructing an image from each of the detail images, of which noise components have been suppressed, for example.
Further, U.S. Patent Laid-Open No. 20020071600 proposes a method for suppressing noise components of each of detail images by calculating noise characteristics based on data representing a radiation dose and switching characteristics of a smoothing filter, applied to each of the detail images based on the noise characteristics.
It is known that the amount of noise components included in a radiographic image depends on spatial frequencies of the radiographic image as well as a radiation dose during radiographic photography when the radiographic image is obtained (If the spatial frequencies are higher, the noise amount is larger).
However, when the level of noise suppression is determined, the spatial frequencies are not considered in the methods disclosed in U.S. patent Laid-Open No. 20020071600 as described above. Therefore, suppression of the noise components in the radiographic images by these methods is insufficient.